The architecture/design of these quantum computers can do arbitrary computations. The specific machines we are currently able to build don't have enough qubits

The architecture/design of these quantum computers can do arbitrary computations. The specific machines we are currently able to build don't have enough qubits that remain coherent long enough to solve the problems we care about. We've built the aeolipile, now we need to scale it up to a steam turbine capable of moving ships and trains.